1. Field of the Invention
This invention relates to the field of rigging. More specifically, the invention comprises an extendable motion-limiting device that has an integral retracting and aligning feature.
2. Description of the Related Art
Rigging devices are used to link one object to another. In many applications, this link must be able to accommodate a varying distance between the two objects. One example is the stabilization of a floating dock, where the water level varies over a significant range. A cable securing such a dock must restrain the dock, yet also extend and retract to allow tidal variations.
FIG. 1 shows a prior art check cable 14 connected between first block 10 and second block 12. Throughout this disclosure, the reader should bear in mind that the term “cable” is intended to encompass any tensile member, and could include fiber ropes, synthetic cables, chains, wire ropes, straps, and similar items. In some applications, the cable could even be a rigid rod.
The check cable is attached to the blocks by any convenient method. The actual method of attachment is not significant to the present invention. Thus, the attachment means could include tying, embedding, potting, swaging, and many other known approaches.
The reader will observe that check cable 14 is slack in FIG. 1. It has deflected downward under the influence of gravity. Slack in a stabilizing line often presents a problem. In the case of a floating dock, the slack can become entangled in other structures or moving watercraft. Thus, it is customary to add features which control the slack.
FIG. 2 shows a check cable 14 incorporating a spring 16 connected to attachment point 18. If first block 10 and second block 12 are moved closer together, spring 16 takes the slack out of the cable by creating slack portion 20. Relative motion between the first and second blocks is thereby governed by two distinct phenomena. As the blocks move further apart from the position shown in FIG. 2, spring 16 will expand and slack portion 20 will become more horizontal. When all the slack is removed from slack portion 20 the check cable will draw taut. The check cable is preferably made of a material having a much higher linear extension coefficient than spring 16. Thus, as the blocks move away from each other, there will be a first range of distance where the force tending to pull the blocks together (a “restoring force”) grows linearly. Then, when the check cable grows taut, the restoring force tending to pull the blocks together will suddenly grow much greater. This combination of force is advantageous in many circumstances.
Of course, there are many known devices for producing such a combination of forces. FIG. 3 shows one such arrangement. The near end of check cable 14 is directly connected to first block 10. The far end is rolled around retracting drum 22, which is attached to second block 12. The retracting drum is biased by a spring so that it tends to “reel in” the free cable. The spring can be a conventional one, so that the restoring force increases as the blocks are moved further apart. On the other hand, a constant force spring can be used in the drum so that the restoring force is constant on the check cable. The end of check cable 14 is solidly connected to the drum so that once a defined extension limit is reached, the check cable draws taut and restricts further motion between the blocks.
FIG. 4 shows another variation on this concept. Planar spring 24 is made of a material which can elastically deform to a state where it is completely straight. The dual extension behavior is thereby realized. A relatively weak restoring force is present at first. This builds steadily until the planar spring is completely straightened, at which point the restoring force rises dramatically.
A parallel arrangement of two dissimilar materials is shown in FIG. 5. Elastic member 26 is an expandable elastic material (such as a piece of natural or synthetic rubber). Check cable 14 is wrapped around elastic member 26. Though the check cable has the appearance of a helical spring, it does not act as a spring. The assembly expands until check cable 14 draws taut, at which point further expansion is restricted. The reader will therefore appreciate that extension limiting devices having changing restoring force characteristics are useful in many applications.